1. Field of the Invention
The present invention generally relates to a driving force control system for a vehicle. More specifically, the present invention relates to a system that controls the driving force in a vehicle equipped with a step-type automatic transmission.
2. Background Information
One known driving force control system for controlling the driving force of a vehicle is disclosed in Japanese Laid-Open Patent Publication No. 4-260853. The driving force control system disclosed in this publication determines a target driving force of a vehicle based on the vehicle speed and the driver""s operation of the accelerator pedal. Accordingly, the driving force control system controls the gear position and the engine torque so as to achieve the target driving force. The engine torque is set for delivering the target driving force established for each gear position by using a chart that provides the amount of throttle opening required. During shifting of the gear, the timings are controlled in view of the response of each control operation. The timings are controlled such that the gear position and the throttle opening are changed at the same time. The timings are controlled such that the start of actual shifting of the transmission and opening of the electronically controlled throttle are synchronized. In this manner, a shock during the shifting can be reduced.
However, when the engine torque is changed at the beginning of an actual shifting operation, such problems as clutch slippage (when torque is being increased) and sudden clutch engagement (when torque is being reduced) can occur easily. As a result, the shifting operation may be delayed or the shifting shock may be worsened. These situations occur because the clutch is unstable at the beginning of the inertia phase of the shifting operation. Accordingly, the balance between clutch capacity and input torque can easily be disrupted.
If the system changes the engine torque at the end of the shifting operation to avoid the aforementioned problems, the change in the engine torque causes an additional effect on the output torque. More specifically, the aim of increasing the engine torque Te at the end of the upshifting operation is to eliminate the sudden decrease in the output torque To. However, if the target engine torque is changed towards the end of the shifting operation in an attempt to eliminate the sudden change in the driving force that occurs before and after the shifting operation, a dip in the output torque To will occur. This is due to a delay in increase of the actual engine torque Te. This delay occurs due to the delay in the control of the electronically controlled throttle. As a result, this dip in the output torque To causes the shifting shock.
In other words, this kind of phenomenon will occur even if the engine torque is controlled so as to change near the end of the shift operation. Therefore, it is preferable to avoid the disadvantageous phenomenon mentioned above when controlling the driving force to achieve the calculated target driving force. Furthermore, it is preferred that the measure for avoiding the phenomenon be more effective in controlling the driving force in a suitable manner.
Additionally, during an upshifting operation in accordance with conventional technology, the gear ratio is shifted toward a higher gear position either because the driver returned his foot to the accelerator, or because a gear ratio abnormality occurred when the target engine torque was being calculated based on the actual gear ratio. If the actual gear ratio calculated from the input and output speeds is solely used in calculating the target engine torque based on the target driving force, then the target engine torque will increase more than necessary. Accordingly, the shifting shock, which is a sharp increase in the output torque, will worsen if the calculated actual gear ratio shifts toward a higher gear position due to an apparent change in the actual gear ratio. This apparent increase in the actual gear ratio is caused by OWC (one-way clutch) release due to the driver""s foot returning to or releasing from the accelerator pedal during shifting or due to a sensor abnormality. In particular, since the target engine torque equals the target driving force divided by the gear ratio, the target engine torque could possibly reach the maximum if the gear ratio keeps decreasing as indicated by the arrow and results in an extremely small value. Accordingly, the output torque undergoes a sharp rise because the hydraulic pressure increases in response to the increase in the engine torque.
In view of the above, there exists a need for a driving force control system, which overcomes the above-mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
The object of the present invention is to provide a driving control system that achieves shift operations without worsening shift shocks.
The aforementioned object can be attained by providing a driving force control system for a vehicle with a step-type automatic transmission. The vehicle driving force control system basically includes a target driving force calculating section, a gear ratio determining section, and a target engine force calculating section. The target driving force calculating section is configured to calculate a target driving force. The gear ratio determining section is configured to determine a calculated engine torque gear ratio based on a current gear position when shifting is not occurring, and based on an actual gear ratio calculated from input and output rotational speeds at least at one point in time during shifting. The target engine force calculating section is configured to calculate a target engine torque based on the target driving force and the calculated engine torque gear ratio.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.